The invention relates to an improved process for the manufacture of vinyl halide polymers in aqueous dispersions.
In all known processes for the polymerization of vinyl halides, optionally together with other monomers, in aqueous dispersion, heavy polymer deposits are formed, to a greater or lesser extent, on the inner surfaces and fittings of the reaction vessel. Because of their considerable thermal insulation properties, those coatings prevent the controlled removal of the heat of polymerization. That causes a problem, especially in large reactors, which, owing to the shorter cycle per unit of product quantity and therefore the smaller amount of work involved, are especially advantageous economically. For safety reasons, polymerization batches can therefore be operated only at low polymerization rates.
The deposits often enter the product, however, and in some cases considerably impair its quality, for example, as a result of specks (fish-eyes) in the finished product.
For those reasons, it is necessary to clean the reaction vessels thoroughly after each charge, using pressurized water-jet devices, or in many cases, by hand by means of spatulas. This is not only uneconomical, but also requires extensive safety precautions to avoid endangering the health of the operating personnel and to avoid introduction of vinyl halide vapors in the plant atmosphere.
It is therefore desirable and necessary, especially from the point of view of hygienic working conditions, to prevent, or at least reduce substantially, such polymer deposits in the reactor. Many attempts have therefore already been made to achieve that aim.
For example, DE-OS No. 27 39 708, corresponding to British Pat. No. 1,554,499, proposes using salts of organic hydroxylated carboxylic acids, such as citrates, lactates and tartrates, as part of the polymerization charge, but deposits on the walls are still not satisfactorily prevented thereby.
The coating of the reactor wall with polar organic compounds, for example, organic sulfur compounds or quinones, together with at least one organic dyestuff and at least one inorganic pigment (DE-OS No. 20 44 259, abstracted in Chem. Abstracts 77, 20374t [1972]) or condensation products of polyhydric phenols, for example, hydroquinones (DE-OS No. 28 26 681, corresponding to U.S. Pat. No. 4,080,173) or of polyaromatic amines (U.S. Pat. No. 4,024,301), is very expensive since those coatings have to be renewed after only a small number of batches and the coating substances can have a detrimental effect on both the reaction and the quality of the product.